1. Field of the Invention
The invention relates to the motor vehicle transmission having gearings for six forward speeds and one reverse speed as are commonly known from modern passenger motor vehicles having advanced stickshift transmission technology incorporated therein.
More particularly, the invention relates to such a transmission with an input or drive shaft with at least one first and one second countershaft, and with an output shaft; the shafts being arranged parallel to one another but only in pairs in each case in a common plane, and gearwheels arranged on the input shaft meshing, in each case in pairs, with gearwheels of the first and second countershaft, respectively. Furthermore, shift bushings or gearshift sleeves for non-rotatably locking one gearwheel of each pair of the respective shaft that carries it are provided, while the respective other gear wheel is non-rotatably locked to the shaft which carries it; also the first and second countershaft each have a gearwheel that meshes with a gearwheel arranged on the output shaft; and lastly two of the pairs comprising a common gearwheel on the input shaft.
2. Description of the Related Art
A transmission of the aforesaid type is known from EP-PS 224 407.
Reduction transmissions of conventional design as usually utilized for motor vehicles with a standard drive configuration--i.e. with an engine arranged at the front, perpendicular to the direction of travel--in most cases use an input shaft on whose coaxial extension the output shaft is located. Located parallel to this input/output shaft is an intermediate shaft or countershaft, and between these two shaft arrangements mesh as many gearwheel pairs as there are gears provided by the transmission. For reverse gear, it is then usual to utilize another idler or reversing gearwheel that is mounted on a further countershaft so as to rotate freely.
However, this conventional reduction transmission design leads to very long transmissions if the transmission is to be provided with a plurality of gears.
On the other hand, the trend in modern automotive design is towards reduction transmissions with increasing numbers of gears, since with greater numbers of gears it is easier to utilize the optimum and therefore most economical speed range of the engine.
A further trend in modern automotive design, moreover, is that of arranging the engine and transmission, as a design unit, perpendicular to the long axis of the vehicle in the engine compartment.
There thus exist in transmission design two conflicting demands, namely on the one hand to provide as many gears as possible in the transmission, but on the other hand to make the axial length as short as possible.
In this context numerous transmissions are known in which not just one, but two or more countershafts are provided, so that the various gearwheel pairs can be arranged in a space-saving manner. In a sectioned depiction in which the various shafts are sectioned radially, the shafts in this instance lie at the vertices of an irregular polygon. This means that in each case only two of the shafts (which of course run parallel to each other) define a common plane, while all the other shafts lie outside this plane.
In the six-speed reduction transmission known from the aforesaid EP-PS 224 407, a total of five mutually parallel shafts are provided, namely an input shaft with a total of six fixed gearwheels, a first countershaft with three freewheeling gearwheels and one fixed gearwheel, a second countershaft with three freewheeling gearwheels and two fixed gearwheels, a third countershaft that carries only one reversing gearwheel for reverse gear, and lastly an output shaft with a freewheeling gearwheel that is connected via a differential to the output shaft. This known six-speed transmission thus comprises a total of seventeen gearwheels on a total of five shafts. The axial length of this transmission is determined by the shaft with the most gearwheels, i.e. in this case by the input shaft with the six fixed gearwheels.
In the known transmission, separate gearwheel pairs are each provided for first, second, third, sixth, and reverse gears, each consisting of a fixed gearwheel on the input shaft and a freewheeling gearwheel on the first or second countershaft. Only in the case of reverse gear is the reversing gearwheel on the third countershaft shifted into place between the fixed gearwheel on the input shaft and the fixed gearwheel on the second countershaft. Common output from the countershafts to the output shaft occurs via the two fixed gearwheels on the countershafts, which simultaneously mesh with the differential gearwheel on the output shaft. Only for fourth and fifth gears is there an arrangement in which a fixed gearwheel on the input shaft meshes with a freewheeling gearwheel on the first countershaft and simultaneously with a freewheeling gearwheel on the second countershaft. Either fourth or fifth gear can therefore be engaged by selectively non-rotatably locking one of these freewheeling gearwheels to the respective countershaft that carries it.
However, for the reasons already mentioned the axial length is extremely great, especially because of the total of six gearwheels required on the input shaft, and the additional gearwheel plane required for output.
EP-OS 207 910 discloses a transmission of similar configuration, but designed only as a five-speed transmission. In this case a separate gearwheel pair is provided for each individual gear, i.e. for the five forward gears and for reverse. With one input shaft, two countershafts, and one output shaft, this creates a need for a total of fifteen gearwheels, of which a total of six are arranged next to one another on the input shaft, so that here again, with respect to the gearwheel planes required for output, the resulting axial length is just as great as with the aforementioned transmission according to EP-OS 224 407, although the transmission according to EP-OS 207 910 is designed for one less gear.
EP-PS 239 553 discloses a further five-speed reduction transmission. This known transmission has only two countershafts, since reverse gear is implemented by directing power via two intermeshing freewheeling gearwheels on the countershafts, one of which has a second gear rim for first gear and operates as a freewheeling gearwheel when reverse gear is engaged, while the other gearwheel on the other countershaft is non-rotatably locked to that same countershaft when reverse gear is engaged.
But even this known transmission, with five forward gears, has a considerable length, since five gearwheels are still located on the input shaft. Thus there is only a slight reduction in axial length with respect to the output gearwheel plane, as compared to the transmissions explained earlier. In addition, this known reduction transmission still requires fourteen gearwheels for five speeds.